To me, intonation is many things:
First: it is the relationship of the tones played by different instruments at the same time. When two trumpets (or other instruments)are playing, sum and difference tones are generated acoustically. When "in tune" those resultant tones line up in frequency and become a constructive part part of the musical presentation. If we play "out of tune", the sum and difference tones become destructive, the end product becomes hard to listen to. If we are talking about playing major chords, the sum and difference tones are all in the same key. A common sense of "intonation" makes large groups of instruments create an "orchestral fabric" that solo instruments can move in and out of.
Second: intonation is the relationship of notes to one another in the same instrument. A piano is tuned for instance "well tempered". This means that moving up the scale, the distance in frequency between the notes is the same. Unfortunately, the sum and difference tones do not line up with well tempered tuning. When a piano plays a C major chord, the resultant tones are NOT in C major. We use well tempered tuning to be able to play in all keys - albeit with equal errors in the resultant tones. The "character" of the key is dramatically diminished. Many harpsichords and organs are NOT tuned well tempered. They are tuned for the job immediately at hand. For a Bach Brandenburg Concerto #2, this would favor F major. Baroque period organs are often tuned similarly as they seldom need a lot of different key signatures.
Of course there is much more to discuss here, but this gets us started.
What I learned in this book was about the compromises necessary when playing - to keep the entire ensemble "happy". Performers can not just let their instruments do what they do, we need to influence pitch depending on its context. I also learned about making resultant tones work for me. Sometimes the resultant tones ABOVE the played notes are more prominent, sometimes the resultant tones BELOW are. In any case, playback systems with great low end extension can make 2 violins sound much more "real" due to difference tones being <50 Hz (A440Hz and Bb 466Hz produce resultant tones 26 Hz and 906Hz for instance).
To "hear" the effects of resultant tones in large groups, grab any Bruckner symphony (Vienna Phil, Furtwängler or Berlin, Karajan is a great start) and listen to the brass. Bruckner composed as if the orchestra was an organ. The brass section plays block chords and incredible resultant tones result.
In small groups, recorder duets make resultant tones very easy to hear. Telemann, Georg Philipp: Blockflötenduette, Label: Raumklang,
EAN: 4035566200409
Order Nr.: RK MA 20040
In relation to perception here, maybe we should take Ravel Bolero. Here Ravel orchestrated instruments together to get a combined sound that was new - unlike the instruments being played. Bach used resultant tones to play notes outside of the range of the organ.
Playback systems with good tonal discrimination, let more of this "goodness" through.
